Electrical power supplies commonly use diode rectifier circuits to convert from alternating current (AC) to direct current (DC), A diode rectifier conducts current only when the input voltage of the rectifier exceeds the output voltage of the rectifier, so a sinusoidal input voltage results in intermittent non-sinusoidal current flow. The intermittent current flow has a primary frequency component equal to the AC input frequency and substantial energy at integer multiples of the AC input frequency (harmonics). Input current harmonics can cause transient current flow in the AC mains, which can increase the power required from the AC mains and can cause heating of the distribution system. In addition, input current harmonics create electrical noise that can interfere with other systems connected to the AC mains.
The power factor of a power supply is the ratio of the real power delivered to a load divided by the apparent input power, where the apparent input power is the Root-Mean-Square (RMS) input voltage times RMS input current. In general, input current harmonics cause the RMS value of the input current to be substantially higher than the current delivered to the load. Many power supplies include power factor correction to reduce input current harmonics. Power factor correction refers to a process to offset or improve the undesirable effects of non-linear electric loads that contribute to a power factor that is less than unity. These effects involve the phase angle between the voltage and the harmonic content of the current. When the voltage and current are in phase, the power factor is unity, but when the voltage and current are not in phase the power factor is some value less than one.